(1) Field of the Invention
The invention relates to an apparatus and EDM method of drilling holes into surfaces without a line of sight from the drilling equipment to the hole location, and more particularly into surfaces of a cast vane cluster.
(2) Description of the Related Art
A gas turbine engine includes a compressor for directing a primary fluid stream axially rearward, through a combustor and into a turbine. The turbine extracts power from a primary fluid stream and transmits the power through a shaft to rotate the forward-mounted compressor. A portion of the primary fluid stream is also directed to one or more secondary fluid streams for use in cooling components of the gas turbine engine. Disposed within the turbine section are alternating, annular stages of rotating blades and stationary vanes. The blades and vanes are disposed circumferentially about a central, longitudinal axis of the gas turbine engine.
Individual turbine vanes are comprised of an inner platform, an outer platform and an airfoil spanning radially outward from the inner platform to the outer platform. The airfoil contains a forward facing leading edge and a rearward facing trailing edge. The airfoil is staggered on the platforms in relation to the primary fluid stream direction, with the airfoil trailing edges of adjacent vanes forming an overlapping array. Together, the platforms and airfoils of adjacent vanes delineate a duct for directing the primary fluid stream rearward. The duct area generally converges in the rearward direction.
Vanes are typically investment cast of high-strength Nickel or Cobalt alloys and may contain multiple airfoils within a single casting. Vane castings with multiple airfoils are referred to as cast vane clusters and have the advantage of reducing the number of inter-platform interfaces in a turbine stage. Inter-platform interfaces are costly to manufacture and are a source of primary fluid stream leakage, which is detrimental to the operating efficiency of the gas turbine engine.
In cast vane clusters requiring cooling, one or more hollow passages extend through the interior of the airfoils for directing a secondary fluid stream. A multitude of cooling holes pass through the airfoil walls and into the hollow passages, allowing the secondary fluid stream to discharge into the primary fluid stream. Preferably, the multitude of cooling holes are drilled from the direction of the airfoil trailing edge and at an acute angle to the cast vane cluster surfaces. The drilling direction and angle are necessary to ensure that the secondary fluid stream is discharged in a substantially rearward direction. This optimizes the cooling effectiveness of the secondary fluid stream and reduces aerodynamic losses in the primary fluid stream.
Typically, cooling holes are drilled after a vane cluster casting is made. The standard methods used for drilling cooling holes in cast articles are laser and electrodischarge machining (EDM). Laser drilling methods utilize short pulses of a high-energy beam, an example is shown in U.S. Pat. No. 5,037,183. Electrodischarge machining (EDM) drilling methods pass an electrical charge through a gap between an electrode and a surface, an example is shown in U.S. Pat. No. 6,403,910. Both the laser and the EDM drilling methods require a line of sight from the drilling equipment to the hole location, limiting the surfaces that may be drilled.
Due to the stagger of the airfoils on the platforms of a cast vane cluster, portions of the duct surfaces are obstructed by the airfoil trailing edges and cannot be drilled using conventional laser or EDM drilling methods. What is needed is an apparatus and method of drilling holes into surfaces without a line of sight from the drilling equipment to the hole location.